A search for the three isomers of cyano-1,3-butadiene in TMC-1: Implications for bottom-up routes involving 1,3-butadiene⋆

The molecule 1,3-butadiene (CH_{2CHCHCH2) could play a key role in the synthesis of the cyclic molecules cyclopentadiene and benzene in cold dense clouds. Since 1,3-butadiene is non-polar, we searched for its cyano derivative, which exists in the form of three different polar isomers, in the cold dense cloud TMC-1. We used the most recent data obtained with the Yebes 40 m telescope in the Q band (31.0−50.3 GHz) in the frame of the QUIJOTE project. We did not detect either of the isomers of 1-cyano-1,3-butadiene, and we derive 3 σ upper limits to their column densities of 1.2 × 10^{10 cm−2 and 2.0 × 1010 cm−2 for E- and Z-1-cyano-1,3-butadiene, respectively. Our results are not consistent with those from , ApJ, 948, 133), who determine a column density of 3.8 × 1010 cm−2 for E-1-cyano-1,3-butadiene in TMC-1 using GBT data and a line stack technique. At the current level of sensitivity of our data, there is tentative evidence for the presence of the third cyano derivative isomer, 2-cyano-1,3-butadiene, although a firm detection must await more sensitive data. We derive an upper limit to its column density of 3.1 × 1010 cm−2. This isomer cannot be formed in the reaction between CN and 1,3-butadiene, according to experimental and theoretical studies, and thus we speculate whether it could arise from neutral-neutral reactions, such as C2 H3+CH2 CHCN and CH2 CCN+C2 H4. From the upper limit on the abundance of 1-cyano-1,3-butadiene derived here, we estimate that the abundance of 1,3-butadiene in TMC-1 is below 10−11−10−10 relative to H2. The low abundance inferred for 1,3-butadiene makes it unlikely that it plays an important role in bottom-up routes to cyclopentadiene and benzene.}}